The present invention relates to a method and apparatus for maximizing the utilization of an electric motor under load and in particular relates to an electronic device that is used to calculate the actual torque being produced by an electric motor at any instant in time and comparing this value of torque ratiometrically either with the maximum (peak) torque that the motor could produce under the line voltage available at that instant or with rated motor torque (the torque the motor can produce without overheating at rated voltage), thus giving an indication of whether the motor is approaching the breakover torque point (the point at which peak torque is available) or is under utilized or over utilized.
Various electrical motors, such as those used with underground mining machinery, operate with wide variations in motor voltage. Typical values are in the range from fifteen percent above nominal down to twenty-five percent below nominal. This is caused by poor highline regulation in the power distribution system to the mine, voltage drop in the power center used to reduce the highline voltage to the operating voltage, and the voltage drop in long electrical cables connecting the motor to the power center. Since the maximum torque that a conventional induction motor can produce is approximated by a second order relationship of actual operating voltage, the motor can produce much higher than rated maximum peak torque when operating with over voltage and much lower than rated maximum peak torque when operating at under voltage. Since it is recognized that electric motor performance is bounded by both horsepower and torque capabilities, it is important that maximum use be made of the torque capability of the motor.
Since the maximum or peak motor torque that an induction motor can produce is quite variable from its nominal or rated value, and because the peak motor torque varies considerably in particular operations with poor voltage regulation such as with coal mining equipment, a given motor torque load may cause the motor to stall when the line voltage is low. This sometimes results in damage to the motor. However, under conditions of high operating voltage, it is possible that the motors are being under utilized because they can produce greater torque which means that production rates have not been maximized.
The present invention provides a method and apparatus for determining the actual torque being produced by a motor and allows this value to be compared to the peak torque that would be available under the instantaneous operating voltage thereby warning of impending motor stall conditions, or allows the measured torque to be compared to the rated torque at a particular line voltage that would be available under the instantaneous operating conditions thereby warning the operator of potential overheating conditions if the actual torque exceeds the rated torque.
Thus, the present invention overcomes the disadvantages of the prior art by allowing maximum use of the torque capability of an electric motor.
It is an object of the present invention to generate signals representing the actual motor torque at a particular instant and comparing it with the rated motor torque and generating a control or warning signal if the calculated actual torque exceeds the rated torque thereby warning the operator of a potential overheating condition.
It is also an object of the present invention to compare the instantaneous motor torque with the peak motor torque for the instantaneous line voltage thereby giving an indication of approaching motor stall to the operator.
It is a further object of the present invention to use the potential overheat warning signals or the approaching stall warning signals to automatically reduce the load on the electric motor thereby preventing damage thereto.
It is still another objection of the present invention to compare the actual torque of the motor with the rated torque available at the instantaneous operating voltage whereby the motor load may be increased if the actual torque is below the rated torque available thereby maximizing the use of the electric motor.